Remove files.Platform model (bug 1062958)

Ref: https://bugzilla.mozilla.org/show_bug.cgi?id=1062958

One point to be highlighted: before this change, we had three way of dealing with platforms, the `Platform` model, the `PLATFORM_*` constants, and the raw `platform_id` attribute. Now we have only two.

Many methods had platforms in their signatures, among those some were using the `Platform` model. So I had the choice, when updating those, to switch to constants or ids instead. I've chosen ids, for the lightness, and because *at the end* we need to give an id to the `File.platform` property. But this can be discussed. :)
We have moved from `ModelChoiceField` to `ChoiceField`, and the first creates an empty choice by default, so `empty_label=None` was here to prevent that, but we don't need it anymore with `ChoiceField`.